I have a 620 type 5 speed, but I don't have a L block front case for it.It had a diesel case on it till I sold it recently, the oil in it wasn't the greatest, it was kinda dark, but there were no loose metal pieces, this transmission actually came from Australia and was used to trot horses at a track.What year is your 620, does it have a short or long shaft 4 speed transmission, because if it has a long shaft, you can use the front case on this transmission.

“The difference between genius and stupidly is that genius has its limits” Albert Einstein

I have never heard of the A block wearing out the dogleg transmission, but then they put the L case on one and put it into the 200SX which worked fine also, but then folks started to use them in their trucks, 5th gear in the dogleg is not made for that type of heavy duty work, it starts to howl in no time at all under load, Dan Hart told me when I bought a Dogleg from him, he said when I load the truck up, use 4th gear, that is all he said, and he was so right.They are fine in cars, but are not a good choice for a truck.

“The difference between genius and stupidly is that genius has its limits” Albert Einstein